The first helical groove rotor in a barrel bore apparatus was probably used to grind meat, long before rubber was invented. The meat grinding operation wedges the material against the barrel bore so as to cause the helical groove to move the material longitudinally downstream.
The extrusion flow of the modern rubber extruder, however is quite different. Adhesion to the barrel surface and resistance to forward flow causes the rubber contacting the barrel to be moved circumferentially across the helical groove, from the leading edge to the trailing edge. Material arriving at the trailing edge is forced downward and follows the path of least resistance back to the leading edge of the helical groove. This flow pattern concentrates the mechanical shear to the material at the barrel and groove surfaces and thereby generates temperature stratified material consisting of warm material surrounding a cold core. My U.S. Pat. Nos. 2,744,287, 3,375,549 and 4,075,712 are designed to and do correct the temperature stratification, to a reasonable blend of worked and overworked process material. The present invention is therefore aimed at a mixing labyrinth which does not develop temperature stratification by the simple means of moving the warmed material downstream quickly and to thereby minimize the rework of already worked material.